1. Field of the Invention
The present invention relates to the field of caller identification (caller ID) and, more particularly, to a device for automatically identifying DTMF and FSK system by activating only a single decoder.
2. Description of Related Art
The service of caller ID has become popular recently. In general, caller ID service refers to that the caller""s identification, such as caller""s telephone number or name, is shown on the screen of caller ID device connected to the telephone. Conventionally, there are two techniques that are implemented for providing the caller ID service. One is FSK (Frequency Shift Keying) specified by Bellcore (Bell Communication Research), which is available in North America. The other one is DTMF (Dual Tone Multi-Frequency) which is generally available in Europe.
In the DTMF system, the transmitting caller number data is the same as the telephone number of a typical keypad telephone. That is, a digit or character of caller""s data is represented by synthesizing a column frequency and a row frequency, in which the row frequency is one of 697 Hz, 770 Hz, 852 Hz, and 941 Hz and the column frequency is one of 1,209 Hz, 1,336 Hz, 1,447 Hz, and 1,633 Hz.
In the FSK system, xe2x80x980xe2x80x99s and xe2x80x981xe2x80x99s of a bit stream are transmitted in different frequencies. For example, in the Bell 202 specification, xe2x80x980xe2x80x99 is a 2,200 Hz signal for {fraction (1/1,200)} second, and xe2x80x981xe2x80x99 is a {fraction (1/1,200)} Hz signal for {fraction (1/1,200)} second. In V.23 specification, xe2x80x980xe2x80x99 is a 2,100 Hz signal for {fraction (1/1,200)} second, and xe2x80x981xe2x80x99 is a 1,300 Hz signal for {fraction (1/1,200)} second. A typical caller ID signal format is illustrated in FIG. 4. As shown, the caller ID signal format comprises a channel seizure signal field, a mark signal field, a message type field, a message length field, a representation layer message field, and a checksum field, based on the time of signal received. The channel seizure signal consists of 300 logical xe2x80x980xe2x80x99s and xe2x80x981xe2x80x99s continuously and alternately arranged. The mark signal is of continuous logical xe2x80x981xe2x80x99s that consists of 180xc2x125 or 80xc2x125 bytes, and is equivalent to 130xcx9c70 ms or 47xcx9c87 ms in terms of time. Therefore, in such a 47xcx9c170 ms time period, there is only a sinusoidal signal having a frequency of 1,200 Hz or 1,300 Hz transmitted in the FSK system. Once a logical xe2x80x980xe2x80x99 is present, it indicates that a message type character is being transmitted. In addition, the message type, message length, representation layer message, and checksum fields are provided for indicating the format, length and content of the caller""s data, respectively. The checksum is provided to check the correctness of the caller""s data.
In various countries, caller ID service is provided by various FSK and DTMF systems. As such, the caller ID device commercially available on the market must satisfy the requirements of both systems, i.e., the caller ID device must be able to determine whether the caller ID signal is of FSK or DTMF format and receive the same.
Such a caller ID device is typically equipped with a FSK decoder and a DTMF decoder. When a caller ID signal is present, both FSK and DTMF decoders are simultaneously turned on for detecting the FSK signal and DTMF signal, respectively. If at least one valid DTMF signal is present for a sufficient period of time, the FSK decoder is turned off. Thereafter, only DTMF signals are received. On the contrary, the two decoders are kept on, and, when receiving logical xe2x80x980xe2x80x99s and xe2x80x981xe2x80x99s that are continuously and alternately arranged, the DTMF decoder is turned off. Thereafter, only FSK decoder is on for receiving FSK signals until a correct checksum is received.
However, one of the major disadvantage of the above caller ID device is such that both FSK and DTMF decoders have to be turned on at the same time. Therefore, the hardware requirement is twice as that of a single decoder. Moreover, the power required is also doubled.
Another conventional caller ID device is provided with a high speed central processing unit (CPU) or digital signal processor (DSP). The FSK and DTMF decoders are thus implemented in software and executed concurrently to detect the caller ID signal. As such, because the CPU executes two sets of software at the same time, the CPU execution time is twice as that in executing a single set of software. In addition, the memory space required is also doubled because memory can not be shared by the two sets of software.
A further conventional caller ID device utilizes a prompt signal to determine whether the caller ID system is of FSK or DTMF. For example, when sending caller ID signal, FSK system typically transmits a long ringing tone in the beginning, and then transmits a FSK caller ID signal between the first and second ringing signals. Alternatively, the DTMF system typically transmits a short polarity inverse signal in the beginning, and subsequently transmits the DTMF caller ID signal. Thereafter, the ring signals are transmitted. Therefore, it is possible to determine to turn on the FSK decoder or DTMF decoder based on the prompt signal. However, in actual application, the relation between the prompt signal and the signaling of the caller ID system may not be so permanent. For example, it is not necessary for the FSK system to send a ringing tone first. The FSK system may use a reverse polarity signal as a prompt signal, even does not have any prompt signal. Therefore, it is not satisfactory to determine the type of caller ID system by the prompt signal.
It is therefore an object of the present invention to provide a device for automatically identifying DTMF and FSK system, which is able to automatically determine the type of the caller ID system by only activating a single decoder.
To achieve the above object, the device of the present invention is provided with a signal detector, a DTMF decoder, a FSK decoder and a microprocessor. The signal detector is provided for detecting signals on a telephone line. The microprocessor is activated when the signal detector detects a signal on the telephone line for only activating the DTMF decoder to calculate frequency spectrum of the detected signal. When the calculated frequency spectrum conforms to DTMF frequency spectrum, it is determined that a DTMF system is in use. When the calculated frequency spectrum only has a frequency of about 1,200 Hz or 1,300 Hz, it is determined that a FSK system is in use, so as to deactivate the DTMF decoder and activate the FSK decoder.
The above and other objects, features and advantages of the present invention will become apparent from the following detailed description taken with the accompanying drawings.